Adjustable sliding brush transformer



Dec. 8, 1964 c. A. NEUMANN ETAL ADJUSTABLE SLIDING BRUSH TRANSFORMER Filed Aug. 9, 1961 4 Sheets-Sheet 1 I8 j. 1 Q\ .79

Il v "i y 26 [mle/6K9. C/arence gil/60mm?, Henry H6/erg Bam/'ceaforaf De- 8, 1964 c. A. NEUMANN r-:TAL 3,150,842

ADJUSTABLE SLIDING BRUSH TRANSFORMER Filed Aug. 9, 1961 4 Sheets-Sheet 2 Dec. 8, 1964 c. A. NEUMANN l-:TAL 3,160,842

ADJUSTABLE sLInING BRUSH TRANsFoRM'ER 4 Sheets-Sheef 5 Filed Aug. 9, 1961 mmm,

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Dec. 8, 1964 C. A. NEUMANN ETAL ADJUSTABLE SLIDING BRUSH TRANSFORMR Filed Aug. 9, 1961 4 Sheets-Sheet 4 United States Patent O This invention relates to adjustable transformers of the type having a high resistance brush slidable along a circular, electrical contact surface on the exterior of the winding, usually connected as an autotransformer, and more in particular to improved means for dissipating the heat generated in the area where the brush touches the winding contact surface.

In sliding brush autotransformers a large amount of heat is generated at the interface where the brush touches the Contact surface of the winding. This causes the interface to be the hottest spot in the autotransformcr, since the temperature in the area of Contact is many times that of the remaining portions of the autotransformer. The result is that the temperature at the interface is the limiting factor in the construction and operation of sliding brush autotransformers because the high temperature will cause insulation to fail, or the winding contact surface to burn out or become deformed before other parts of the autotransformer are damaged.

The core of a sliding brush autotransformer operates at a much lower temperature than the windingbrush interface. Consequently, the core is theoretically capable of acting as a heat sink to remove heat from the high temperature interface. However, the core is prevented from being an eficient heat sink by the manner in which sliding brush autotransformers are conventionally made. The winding of such autotransformers is a toroidal coil wound on an annular core. The winding must be raised near the outer circumference of the core to furnish a surface that can be machined fiat to form the contact surface that the brush slides over. The winding is usually raised by placing on the core a plastic form member having a flat, thickened outer circumference, and an insulated wire conductor Iis coiled around the form member. This causes the thickened portion of the form member to be interposed between the winding contact surface and the core. Thus the winding and core are separated by a good thermal insulating barrier in the area where the greatest amount of heat is generated, and the core cannot function elliciently as a heat sink for the high temperature interface.

The limiting effect of the high temperature interface is that sliding brush autotransformers must be operated at a much lower load current level than would be permissible for the remaining materials and elements employed. This results in the size, weight, and cost of the apparatus being increased above that which would be necessary for the thermal and electrical conditions encountered in other pants thereof. Another drawback is that the limiting eect of the high temperature interface prevents the use of higher classes of insulation; the reason is that the winding-brush interface will fail because of the high temperature encountered there before the remaining portions of the apparatus will be raised to temperatres high enough to cause an electrical insulation failure.

lt has long been known in the prior art that the above problems could be solved if eilicient means could be devised for dissipating the heat generated at the interface between the brush and winding. Numerous solutions have been proposed with this end in mind. For example, fans or blowers have been employed to cool autotransformers; thermal conductors have been placed in the brush to carry heat therefrom; and the heat absorbing capacity of 3,160,842 Patented Dec. 8, 1964 the core has been improved. Another prior art arrangement for increasing the heat dissipated at the brush surface is to place brush-engaged contacts at a location remote from the windings and core. The above expedients suffer from the disadvantages that either they are ineffective in reducing the temperature at the brush interface an amount sufficient to eliminate the problem, or else -they result in an increase in the cost, size, and weight of the apparatus that outweighs any increase heat dissipation sufficiently to render tl e expedient impractical for commercial embodiments of the apparatus.

Our invention is based on the realization that the means for removing heat from the high temperature interface need not necessarily dissipate heat directly to the atmos phere. instead, the heat removing means need merely absorb heat from the hot-spot area adjacent the interface and carry the heat to other lower temperature areas of the autotransformer. This results in more uniform temperature distribution throughout the autotransformer, and enables an autotransformer core and winding assembly to use higher temperature insulation and to be operated at greater load capacities for a given size.

We accomplish the above desirable results by employing a heat sink beneath the winding in the area adjacent the winding contact surface. A related heat sink arrangement requiring a split tube has been proposed in United States Letters Patent 2,463,l05. However, the arrangement taught by that patent suers from the disadvantages that the split tube surrounding the core increases the size and cost of the apparatus beyond practical commercial limitations, and the heat absorbing capacity of the heat sink is not uniform for all brush positions in that the area of the tube usable for effectively conducting heat is greatly reduced when the brush is at either end of the winding. Another disadvantage is that the orientation of the tube with relation to the flux in the core requires the tube to be split throughout its length so that it will not be a short-circuited turn.

Accordingly, one object of our invention is to provide an improved heat sink for dissipating heat generated at .the high temperature interface between an autotransformer brush and winding so as to solve problems previously encountered by the prior art without incurring the disadvantages of prior art heat sinks.

A more general object of the invention is to provide an improved sliding brush autotransformer operable at higher load levels for any given size and weight of its core and winding assembly.

Another object is to improve adjustable autotransformer construction so that the use of relatively high temperature insulation is practical.

Another object is to provide sliding brush autotransformers with a heat sink that prevents burn-out or deformation of the Winding contact surface at relatively high load levels.

A further object of the invention is to provide an adjustable sliding brush autotransformer with a heat sink that has the same heat absorbing effect for all locations of the brush on the winding contact surface.

Another object is to provide adjustable autotransformers with a heat sink that is an integral metal body having continuous outer surfaces, yet is not a short-circuited turn about the autotransformer core.

A further object is to provide a heat sink arrangement for sliding brush autotransformers that eliminates the winding brush interface temperature as the limiting factor in the construction and operation of the autotransformer,

A further object is to provide a heat sink in a sliding brush autotransformer that furnishes a fiat surface to aid in fabricating the winding contact surface over which the brush slides.

A further object is to provide a sliding brush autotransformer in which thermal insulation between the winding and core is reduced to the extent that the core becomes an eicient heat sink for the heat produced at the high temperature interface between tde brush and winding.

Another object of the invention is to provide a heat sink for sliding brush autotransformers in which eddy current losses in the heat sink are minimized.

Another object of the invention is to improve adjustable autotransformers of the type having a winding coiled around an insulating form that often has an uneven surface by providing a heat sink that furnishes a llat, even surface for the winding to be wound on in spite of unevenness in the insulating form.

Other objects and advantages of the invention will become apparent from the drawing, specication, and claims, and the scope of the invention will be pointed out in the claims.

Brietly stated, according to one aspect of our invention, an adjustable autotransiormer of the type having a substantially toroidal winding of successively disposed adjacent turns may have a circular electrical contact surface on the winding in a plane substantially perpendicular to the longitudinal centroidal axis of the winding. The winding contact surface provides a circular, commutator-like path for sliding electrical contact between a high resistance brush connectible to an external circuit and successive turns of the winding. Under many operating conditions suiiicient heat is produced at the area of contact between the brush and winding contact surface to damage parts of the autotransformers. Our invention resides in the improvement that prevents damage from excessive heat at the area of contact between the brush and winding contact surface by the use of a substantially circular metal heat sink ring having its central axis substantially coincident with the longitudinal centroidal axis of the winding. The heat sink is substantially enveloped by the toroidal winding and has a flat, even outer surface located in a plane substantially parallel to the previously mentioned plane of the Winding contact surface. The outer surface ot the heat sink is in heat transfer relationship with the Winding contact surface throughout the circular path of sliding movement of the brush, Thus effective distribution of heat into the heat sink and away from the area of contact is achieved for all positions of the brush along its circular path of movement.

In the drawing:

FIGURE l is an isometric, partially cross-sectional, schematic view of an adiustable sliding brush autotransformer in accord with the teachings of our invention.

FIGURE 2 is an enlarged, fragmentary, schematic cross-sectional view of the heat sink embodiment of FIG- URE l.

FlGURE 3 is an enlarged fragmentary, schematic cross-sectional View, corresponding to FIGURE 2, illustrating another embodiment ofthe invention.

FIGURE 4 is an enlarged, isometric, partially brokenaway, schematic view illustrating7 still another embodiment of the invention.

FIGURE 5 is an isometric, partially cross-sectional, schematic View illustrating a further embodiment of the invention.

FIGURE 6 is an enlarged cross-sectional View taken generally along the line 6 e in FIGURE 5.

FGURE 7 is a cross-sectional fragmentary, schematic view on an exaggerated scale illustrating a deficiency of a prior art arrangement.

FIGURE 8 is a cross-sectional, fragmentary, schematic view on an exaggerated scale of the embodiment of FlG- URES l and 2, illustrating an advantage of our invention when compared with the prior art arrangement of FIG* URE 7.

Turning now to the drawings, and more particularly to CII FIGURES l and 2, therein is illustrated an embodiment of an adjustable sliding brush autotransformer l@ in accord with the teachings of our invention. According to conventional practice, the autotransiormer T tl has an annular core lll of laminated form made by spirally Winding a ribbon or sheet of magnetic iron or steel upon itself. T he core ll is enclosed in two generally U-shaped winding forms l2, made from insulating material, such as plastic. The forms may have serrations (not illustrated) in their outer edges to provide guides for placing and holding the insulated conductor forming the toroidal winding t3. rihe upper form l2 may have a thickened portion at its outer circumference to ensure that the outer circumference of the winding 3 will be raised above the inner circumference of the winding; this ensures that insulation will be removed only from the raised portion of the Winding when the commutatorlike surface l5 is fabricated by methods described in paragraphs that follow.

The winding i3 is made from an insulated wire coiled about the core lll to provide a plurality of successively disposed adjacent turns. An outer surface of the winding llll is provided with commutator-like, electrical contact surface it. The surface i5' lies in a place substantially perpendicular' to the longitudinal centroidal axis 23 of the winding, and provides a circular path for sliding electrical contact with a high resistance brush ld. The surface l5 may be formed by removing the insulation from the surface of the wire forming the winding l by grinding the wire -lat, so as to provide a relatively smooth surface for travel of the brush lo. An alternative arrangement is to provide on Athe previously ground flat contact surface, a layer or any of the Well known materials usable tor brush commutating surfaces. The materials employed are usually high corrosion resistance, low electrical resistance metals or alloys. A more detailed disclosure of materials that have long been used in the art for this purpose may be found in standard textbooks, such as Electrical Contacts, by L. B. Hunt, published in 1946, by lonnson, Matthey 8L Co., Ltd.

The brush 16 is conventionally made from a high resistance carbon or graphite type material to prevent excessive currents when adjacent turns of the winding are siort circuited. The brush may be mounted in a holder j' secured to a radiator plate i5, which is mounted on a rotatable shaft :t9 extending through the opening in the core. The brush i6 is urged against the surface 15 by a spring Ztl so as to maintain a predetermined pressure therebetween. The shaft i9 may have a knob 21 attached thereto to facilitate manual turning of the shaft i9 to vary the position of the brush lo on the surface l5. The radiator plate ld aids in dissipating heat generated by the autotransformer particularly that generated in the brush. The core and winding assembly may be supported on a base 2?..

To operate the autotransformer lll, the ends of the winding i3 are connected to an external source E of alternating current, als for example, by plugging them in to an ordinary volt outlet, as indicated schematically by the leads 2d and 25. The output terminals 26 and 27 of the autotransformer are connected respectively to the brush lo and one of input leads 2d, as indicated schematically in FIGURE l. An external load ZS can be connected across the output terminals 2o and 27, and the voltage and current received by the load are determined by the position of the brush le on the commutator-lil'e Contact surface l5 ofthe Winding i3.

As is well known in the art, the heat generated at the interface between the brush llo and the contact surface l5 is very great, making the area of Contact by tar the hottest spot in the autotransformer. This has resulted in autotransiormers being insulated with the lowest type of electrical insulation acceptable for these purposes (Class A insulation), so that only a 55 C. rise of average Winding temperature over ambient temperatures is provided areas/ia i?) for. In prior art autotransformers the use of higher classes of insulation, which would enable the autotr formers to withstand higher temperature rises, and consequently greater loads for any given size, have not been used because the high temperature at the interface causes deterioration of the uninsulated surface lf3, melting of the form l2, rais-ing of winding turns under the brush, or burning out of the insulation on the winding lf2 in the area adjacent the brush to, before the electrical load on the apparatus causes failure of the insulation in other areas. The overall result is that prior art autotransiormer winding and core assemblies have been operated at much lower load levels than they could be used for, if the high temperature at the interface were lowered.

To accomplish the above-described desirable result, autotransformers in accord with our teachings employ a substantially circular, ring-like, metal heat sinh beneath the winding 13 in the area immediately adjacent the winding contact surface l5. The longitudinal centroidal axis of the heat sink should be substantially coincident with the longitudinal centroidal axis 23 of the autotransformer winding to ensure a symmetrical relationship. in the embodiments of FEGURES l and 2, the heat sinlt is a ring Si?, which is made from a metal having a relatively high ythermal conductivity and a relatively low electrical rcsistance. The ring 3@ should be at least as wide as the brush 16. it has been found that silver, copper, and aluminum possess desirable properties in the above given order of preference. Other metals having lower thermal conductivity also may be employed. Alloys of the above mentioned metals are usable as the heat sini; material, and in particular brass and bronze are higluy suitable, lt is also possible to employ a metal plated with a different metal, as for example, brass plated with silver will provide the high conductivity of silver on its exterior surface without the expense of a ring of solid silver. A very thin layer of electrical insulation shouldV be placed between the winding 13 and the heat sinlc 3i). For example, when the heat ysinh 30 is made from aluminum, it has been found that an anodized coating several mils thicl; prevides satisfactory electrical insulation.

The use of a substantially circular, ring-like heat sink, such as 30, provides the additional advantage that heat will be conducted substantially equally in both directions away from the high temperature interface between the brush 16 winding contact surface 2.5 regardless of the location of the brush along its path of movement. The reason is that because the ring is continuous and circular, there will be a continuous metal path on both sides of the brush to conduct heat in both directions circumferentially around the ring no matter where the brush is located.

Another advantage of a heat sink arrangement in accord with our teachings is that the heat sink cannot act as a short-circuited turn about the flux in the core. in prior art heat sink arrangements, complicated and expensive constructions had to be employed in order to prevent the heat sink from becoming a short-circuitcd turn about the ux in the core, since this would have caused the heat sink to burn up in a very short time. In our arrangement the outer surfaces of the heat sink define a closed metal polygon in a plane that is parallel to the cross-sectional plane of the core through which the iiux travels; this construction provides a large mass of metal for absorbing heat. However, the closed metal polygon does not go around the core, and thus cannot be a shortcircuited turn. This relationship is illustrated in Fl@- URE l wherein the cross-sectional plane of the core through which the llux travels, and the plane in which the outer surfaces of the heat sini; delne a closed metal polygon are both the plane in which the core il and heat sink 30 are shown in cross section. The last-mentioned plane is substantially perpendicular to the plane of the contact surface l in which the brush llo makes electrical contact with the winding 13.

rib-

There is bound to be some eddy current loss in the heat sink because of the stray iux from the windings and core. However, the metal surface area in the abovementioned cross sectional plane of the heat sink, in which the eddy currents are generated, is relatively small. Consequently the eddy current losses in the heat sink are lzept at a low level. Furthermore, autotransiormers employing our improved arrangement can bc operated at much higher temperatures than those constructed according to the prior art. This reduces eddy current losses even further, since the resistance of the heat siir goes up as its temperature rises.

ln the embodiment of FIGURE 3, the structure of the autotransformer is identical to that shown and described with reference -to FGURES l and Z, except that the heat sini: is a laminated structure. The heat sink 35 can be formed by winding a length or strip of high conductivity metal about itself to form a spiral, or it can be made by placing a plurality of circular rings concentrically about each other. This type of structure reduces eddy current losses because eddy currents can only be formed in each individual lamination Sie and not in the entire cross sectional area of the heat sink.

FGURE 4 illustrates another embodiment of the invention in which the autotransformer is identical to that disclosed with reference to FGURES l and 2, except for the structure of the heat sinh 4h. The heat sink iti has integral therewith a plurality of projections 4l that extend through the insulating form member l2 into Contact with the core il. This arrangement provides a metal path bc- 'twecn the winding contact surface l5 and the core lll through which heat can be conducted to the core that is broken only by the thin layer of insulation between the winding i3 and heat sink dil. Since the core is at a lower temperature than the winding-brush interface, it acts as a mass of metal providing an additional heat sinh mass for carrying heat away from the interface. The embodiment of FIGURE 4 can be manufactured by drilling holes in the form member l2 for the projections el to extend through, or the form member l2 can be cast around the heat sini; di).

FIGURES 5 and 6 illustrate another embodiment of the invention in which the core itself serves as the heat sink. ln this embodiment, the core :itl is formed by winding a tapered strip of core material in spiral fashion about itself. rThe radially inner end El of the strip is smaller than the outer end S2 and gradually increases in width until a predetermined point 53 is reached, after which the strip is of continuous width. This arrangement provides a flat even upper surface 5d on the core Sti on which the winding contact surface i5 can be fabricated. 0nly a predetermined minimum amo-unt of electrical insulation 55 is provided between the winding i3 and thc upper surface 534i of the core, so as to prevent electrical breakdown but permit good heat transfer therebetween. For example, where the electrical stresses encountered are up to about G volts, a coating of plastic insulation such as an epoxy resin about 5 mils thick provides satisfactory electrical insulation; the thin coating also permits relatively good heat transfer between the surface le and the core Experiments have shown that when our heat sini-r arrangement is employed, the i igh temperatures at the vinding-brush interface do not cause destruction of the surface l5' or insulation in the of contact before the winding insulation fails in other areas. ln the experiments, autotransformers with and without aluminum heat sinks of the type shown in FiGURES l and 2 were run at overload currents of more than twice their rated load capacity until failure occurred. lin those autotransiormers having a heat sink, the failures occurred in the winding insulation, with no failures occurring at the area of contact between the brush and winding. In those autotransforrners without heat sinks, the failure occurred in the area of contact before the winding insulation failed.

alertes-a These experiments are believed to prove that by ernploying a heat sink in accord with our teaclnngs, the high temperatures at the interface between the brush and winding Contact surface will not be the limiting factor in the construction and operation of the autotransformers. Instead, the electrical insulation in other parts of the apparatus is the limiting factor, and the load capacity of the autotransformer can be increased by utilizing higher classes of insulation. This was not practical in prior art devices.

During construction of commercial embodiments of sliding brush autotransformers employing heat sinks of the structure shown in FGURES l and 2, an unexpectedly beneficial result was discovered which eliminated a problem encountered in another area. This can be explained by reference to FlGURES 7 and 8. FlGURE 7 illustrates a problem encountered in making prior art auto transformers in which no heat sink is employed, beneath the winding 13. ln fabricating the autotransformer, the wire 59 forming the winding i3 was coiled around the core lll after the insulating form l2 had been placed thereon. After this was done, an abrasive wheel di? was employed to remove insulation from the winding and to grind flat the upper surface of adjacent turns to produce the winding contact surface l5. The dotted line dit illustrates what the orientation of the surface l5 would be, if the upper surface of the thickened portion lll which supports the wire SQ, were truly dat. However, it has been found that a high percentage of the form l2 slant or droop slightly at the outer circumference of the thickened portion ll so that they provide a supporting surface on which the wire 53 is wound that slants, as indicated by the dotted line 62. The result is that when the slanting surface on the portion lill is cut by tne abrasive wheel di), the wire 59 is cut more deeply at its circumferentially inner end than at its outer end This reduces the cross-sectional area of the wire more than necessar, which greatly increases its resistance and reduces its mechanical strength. The result is that the winding will frequently burn out at a relatively low load level, or in some cases, the abrasive wheel will completely cut through the Wire. The same problem occurs when the inner end of the form l2 slants below the outer end, except that in this case the outer end d5 of the wire 5i? is cut more deeply than its inner end 64.

Turning now to FIGURE 8, the arrangement of parts is the same as in FIGURE 7 except that the form l2 has a groove ed therein for receiving the heat sink 3l). The circumferentially outer end of the form l2 may droop or slant slightly, so that it deviates from a truly flat surface by the difference between the lines dll and d2, as in the prior are arrangement of FGURE 7. However, regardless of the amount of droop or unevenness in the upper surface of the thickened portion lll, the ring-like heat sink 3ft will seat itself in the groove as soon as one corner, such as 67', touches part of the groove. The circular ring-like structure employed is very stiff, and consequently the heat sink Sil will not be deformed so as to conform to the contour of the groove rflierefore, `the wire 59 will be wound over the flat even surface d on the top of the heat sink in a plane substantially perpendicular to the longitudinal centroidal axis of the winding. When the abrasive wheel dll removes insulation and attens adjacent turns, it will cut evenly through the wire 59 and will not produce portions with greatly reduced cross-sectional areas as is the prior art arrangement in FIGURE 7. Thus, by practicing our invention an unexpected result was achieved in an area not related to heat transfer, and a problem encountered in prior art embodiments was solved.

It has thus been shown that by the practice of our invention, the high temperature generated at the interface between the brush and commutator-like contact surface of the winding has been eliminated as the critical feature in constructing and operating adjustable sliding brush autotransformers. The high temperatures previously encountered are greatly reduced because an effective heat sink is provided for carrying heat away from interface. This permits the use of higher classes of insulation, which turn permits an operating temperature rise of higher magnitude for the autotransformer. The result is that for any given size of core and winding assembly, the autotransformer can be operated at greater loads than were previously possible for the given size.

lt will be understood, of course, ythat while the forms of the invention herein shown and described constitute preferred embodiments of the invention, it is not intended herein to illustrate all of the equivalent forms or ramiiications thereof. For example, those skilled in the art `will realize that our invention is also applicable to those embodiments of sliding brush at autotransformers wherein the brush slides along an electrical contact surface on the side of the winding, in contrast to the illustrated ernbodiments where the contact surface is on the top of the winding. lt will also be understood that the words used are words of description rather than of limitation, and that various changes may be made without departing from the spirit or scope of the invention herein disclosed, and it is aimed in the appended claims to cover all such changes as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. ln an adjustable transformer of the type having a substantially toroidal winding of successively disposed adjacent turns of wire, said winding having on the exterior thereof a circular electrical contact surface that provides a circular path for sliding electrical contact between a high resistance brush connectable to an external circuit and successive turns of said toroidal winding, and suflicient heat being produced at the area of contact between said brush and contact surface to damage parts 0f said transformer, the improvement that prevents damage from excessive heat at said area of contact comprising a closed continuous substantially circular ring-like metal heat sink having its central axis substantially coincident with the central axis of said toroidal winding, the axial thickness of the heat sink ring being at least several times the thickness of said Wire, said heat sink being enveloped by said toroidal winding and having a flat even outer surface in heat transfer relationship with said contact surface and closely adjacent the inner surface of the winding opposite to said contact surface throughout the circular path of sliding movement of said brush, whereby heat produced at any area where said brush contacts said contact surface will be substantially equally distributed throughout said heat sink in opposite directions away from the area of contact for all positions of said brush along its circular path of movement.

2. ln an adjustable autotransformer of the type having a substantially toroidal winding of successively disposed adjacent wire turns having thereon a circular electrical contact surface in a plane substantially perpendicular to the longitudinal centroidal axis of said winding, said contact surface providing a circular path for a sliding electrical contact between a high resistance brush connectible to an external load circuit and successive turns of said winding, and sufficient heat being produced at the area of contact between said brush and Contact surface to damage parts of said autotransforrner, the improvement that prevents damage from excessive heat at said area of contact comprising a closed substantially circular metal heat sink ring having its central axis substantially coincident with said longitudinal centroidal axis of said toroidal winding, the axial thickness of the heat sink ring being at least several times the thickness of said wire, said heat sink being substantially enveloped by said toroidal winding and having a ilat even outer surface located in a plane substantially parallel to said adsense plane of said contact surface in heat transfer relationship with said contact surface and closely adjacent the inner surface of the winding opposite to said contact surface throughout the circular path of sliding movement of said brush, whereby heat produced at any area Where said brush contacts said contact surface will be effectively distributed throughout said heat sink away from the area of contact for all positions of said brush along its circular path of movement.

3, The invention defined in claim 2 in which said heat sink is made from a metal having relatively high thermal conductivity, and relatively low electrical resistance.

4. The invention defined in claim 2 in which said heat sink is made from a metal selected from the group consisting of copper, aluminum, silver, and alloys thereof.

5. The invention defined in claim 2 in which said heat sink is made from a strip of metal convolutely wound to provide a plurality of contiguous laminations that minimize eddy current losses in the heat sink, the plane of the turns of said heat sink being perpendicular to the plane of the turns of said winding,

6. The invention defined in claim 2 in which said toroidal winding substantially envelops an annular core made from magnetic metal, said heat sink and core being separated by a layer of non-metallic electrical insulating material that also thermally insulates the core from the heat sink, and said heat sink having metal projections extending through said insulating material at a plurality of locations into contact with said core so as to provide an essentially unbroken metal path for conducting heat from said area of contact to said core.

7. In an adjustable autotransformer of the type having a substantially toroidal winding of successively disposed adjacent turns of wire substantially enveloping an annular core, said toroidal winding having thereon a circular electrical contact surface in a first plane suhstantially perpendicular to the longitudinal centroidal axis of said toroidal winding, said contact surface providing a circular path for sliding electrical contact between a high resistance brush connectible to an external load circuit and successive turns of said toroidal winding, sufticient heat being produced at the area of contact between said brush and contact surface to damage parts of said autotransformer during operation of said autotransformer, said toroidal Winding producing magnetic flux traveling through said core in a substantially circular path on the inside of the toroid, and essentially all of the iiux in said toroid passing through a second plane substantially perpendicular to said first plane, the improvement that prevents damage from excessive heat at said area of contact comprising a continuous heat sink ring between said core and toroidal winding made of metal having relatively high thermal conductivity and relatively low electrical resistance, said heat sink having its central axis substantially coincident with said longitudinal eentroidal axis, the axial thickness of the heat sink ring being at least several times the thickness of said wire, said heat sink defining a flat even substantially circular ring-like metal surface located in a third plane substantially parallel to said first plane, said metal surface being in heat transfer relationship with said contact surface throughout the circular path of sliding movement of said brush, whereby heat produced at any area. where said brush contacts said contact surface will be effectively distributed throughout said heat sink away from the area of contact for all positions of said brush along its circular path of movement, and the outer surface of said heat sink defining a closed metal polygon in a fourth plane substantially parallel to said second plane, said closed metal polygon being outside the path of the linx in said core.

8. An adjustable sliding brush autotransformer comprising a substantially toroidal winding of successively disposed adjacent wire turns having thereon a circular electrical contact surface in a plane substantially perpendicular to the longitudinal centroidal axis of said winding, said contact surface providing a circular path for sliding electrical contact between a high resistance brush and successive turns of said windings, an annular core of magnetic metal substantially enveloped by said winding, a shaft extending through a hole in the center of said core, said shaft being rotatably mounted in a base supporting the core and winding assembly, a metal radiator plate being attached to said shaft so as to be rotatable therewith, said brush being supported by said radiator plate and rotatable -therewith along said circular path, a closed substantially circular heat sink ring made of metal of relatively high thermal conductivity and relatively low electrical resistance, said heat sink having its central axis substantially coincident with said longitudinal centroidal axis of said winding and being enveloped by said toroidal winding, the axial thickness of the heat sink ring being at least several times the thickness of said wire, said heat sink providing a liat even outer surface located in a plane substantially parallel to said plane of said contact surface in heat transfer relationship with said contact surface and closely adjacent the inner surface of the winding opposite said contact surface throughout the circular path of sliding movement of said brush, whereby heat produced at any area Where said brush contacts said contact surface will be effectively distributed throughout said heat sink away lfrom the area of contact for all positions of said brush along its circular path of movement.

9. In an adjustable sliding brush transformer, a toroidal magnetic core, a circular insulating end cap over an end of Isaid core, said cap having an outwardly open circular channel in its outer periphery, a closed continuous metal ring having a flat outer surface fitted into said channel, a toroidal coil of insulated Wire electromagnetically linking said core and wound spirally over said end cap and metal ring, the outer portions of the turns of said Wire where they overlie said metal ring having their insulation removed so as to constitute a circular brush track, the flat surface of said ring underlying said circular .brush track constituting a planar support for said circular brush track, and an electrical brush wide enough to bridge across at least two adjacent turns of said wire and movable along said brush track, said metal ring additionally constituting a symmetrical heat sink for conducting localized heat developed by the action of said brush in bridging adjacent turns away from the brush area equally in circumferentially opposite directions for all positions of said brush on said brush track, said metal ring having a thickness several times the thickness of said wire.

l0. In an adjustable sliding brush transformer, a toroidal laminated magnetic core of convolutely flatwise Wound magnetic strip material, a pair of mating electrical insulating material circular end caps of rectangular channel shaped cross section fitting axially over and enclosing said core, one of said caps having a circular web portion with an outwardly open circular groove in its outer periphery, a flat outer surfaced nonmagnetic continuous metal ring fitted into said groove, a toroidal coil of insulated wire electromagnetically linking said core and Wound spirally over said plastic end caps and metal ring, the outer portions of the turns of said wire where they overlie said metal ring having their insulation removed so as to constitute a circular brush track, the flat outer surface of said ring underlying said circular brush track constituting a planar support for said circular brush track, and an electrical brush wide enough to 'bridge across at least two adjacent turns of said wire and movable along said brush track, said metal ring `additionally constituting a symmetrical heat sink for conducting localized heat developed by the action of said brush in bridging adjacent turns away from the brush area equally in opposite directions for all positions of ancona-a lll said brush on said brush track, said metal ring having a thickness several times the thickness of said wire.

11. In an adjustable sliding brush transformer, a rectangular cross section toroidal magnetic core of convolutely atwise Wound uniform width magnetic strip material, a pair of mating molded plastic circular end caps of rectangular channel shaped cross section iitting axially over and enclosing said core, one of said caps having a circular web portion with an outwardly open circular channel in its outer periphery of rectangular cross section, a rectangular cross section at surfaced closed metal ring fitted into said channel, a toroidal coil of insulated Wire electromagnetically linking said core and wound spirally over said plastic end caps and metal ring, the outer portions of the turns of said wire where they overlie said metal ring having their insulation removed so as to constitute a circular brush track, the flat surface of said ring underlying said circular brush track constituting a planar support for said circular brush track, and an electrical brush wide enough to bridge across at least two adjacent turns of said wire and movable along said brush track, said metal ring additionyally constituting a symmetrical heat sink for conducting localized heat developed by the action of said brush in bridging adjacent turns away from the brush area equally in circumferentially opposi-te directions for all positions of said brush on said brush track, said metal ring hav- `ing a thickness several times the thickness of said wire and being laminated. in a direction perpendicular to the direction of mangetic ux therein produced by said coil.

12. In an adjustable sliding brush transformer, a rectangular cross section toroidal main magnetic core of convolutely flatwise Wound uniform Width magnetic strip material, a pair of mating molded plastic circular end caps of rectangular channel shaped cross section fitting axially over and enclosing said core, one of said caps having a circular web portion with an outwardly open circular groove in its outer periphery of rectangular channel shaped cross section, a rectangular cross section dat magnetic metal continuous ring auxiliary magnetic core tted into said groove, a toroidal coil of insulated wire electromagnetically linking said main core and wound spirally over said plastic end caps and metal ring auxiliary core, the outer portions of the turns of said wire Where they overlie said metal ring auxiliary core having their insulation removed so as to constitute a circular -brush track, the flat planar surface of said ring underlying said circular brush track constituting a planar support for said circular brush track, and an electrical brush wide enough to bridge across at least two adjacent turns of said wire and movable along said brush track, said metal ring additionally constituting a symmetrical heat sink for conducting localized heat developed by the action of said brush in bridging adjacent turns away from the brush area equally in circumferentially opposite ydirections for all positions of said brush on said brush track, said metal ring having a thickness several times the thickness of said wire and being laminated in a direction perpendicular to the direction of magnetic iiux therein produced by said coil.

References Cited by the Examiner UNTED STATES PATENTS 2,009,013 7/35 Karplus et al 336-148 2,204,623 6/40 Ruben 338--159 X 2,463,105 3/ 49 Henniker 366-61 2,949,592 8/60 Smiley 336--148 JOHN F. BURNS, Primary Examiner. 

1. IN AN ADJUSTABLE TRANSFORMER OF THE TYPE HAVING A SUBSTANTIALLY TOROIDAL WINDING OF SUCCESSIVELY DISPOSED ADJACENT TURNS OF WIRE, SAID WINDING HAVING ON THE EXTERIOR THEREOF A CIRCULAR ELECTRICAL CONTACT SURFACE THAT PROVIDES A CIRCULAR PATH FOR SLIDING ELECTRICAL CONTACT BETWEEN A HIGH RESISTANCE BRUSH CONNECTABLE TO AN EXTERNAL CIRCUIT AND SUCCESSIVE TURNS OF SAID TOROIDAL WINDING, AND SUFFICIENT HEAT BEING PRODUCED AT THE AREA OF CONTACT BETWEEN SAID BRUSH AND CONTACT SURFACE TO DAMAGE PARTS OF SAID TRANSFORMER, THE IMPROVEMENT THAT PREVENTS DAMAGE FROM EXCESSIVE HEAT AT SAID AREA OF CONTACT COMPRISING A CLOSED CONTINUOUS SUBSTANTIALLY CIRCULAR RING-LIKE METAL HEAT SINK HAVING ITS CENTRAL AXIS SUBSTANTIALLY COINCIDENT WITH THE CENTRAL AXIS OF SAID TOROIDAL WINDING, THE AXIAL THICKNESS OF THE HEAT SINK RING BEING AT LEAST SEVERAL TIMES THE THICKNESS OF SAID WIRE, SAID HEAT SINK BEING ENVELOPED BY SAID TOROIDAL WINDING AND HAVING A FLAT EVEN OUTER SURFACE IN HEAT TRANSFER RE- 